The present invention relates to a reflex camera with a motor driven reflex mirror movable between a viewing position and a photographing position, and more particularly relates to a reflex camera in which a motor driven reflex mirror is controlled in motion in accordance with its position.
Reflex cameras, as is well known, have a movable reflex mirror (which is hereinafter referred to as a mirror for simplicity) for reflecting light from a subject to a focussing screen of a viewing system. The mirror is adapted to move to a photographing position wherein the mirror is out of the light path of the camera to allow a film in the camera to be exposed.
As a well-known reflex camera, there is a large format of reflex cameras using 120-type film in which a between-the-lens shutter is incorporated therein. In this camera, because the between-the-lens shutter is open when viewing, it is necessary to provide a light-blocking plate between the mirror and an exposure aperture in order to prevent the film from being exposed to stray light. The mirror and light-blocking plate are first swung up to a photographing position after the between-the-lens shutter has been closed as a result of a shutter release operation, then the between-the-lens shutter is again opened and closed to complete an exposure. After the completion of exposure, the swung-up light-blocking plate and mirror return, in this order, to their original or viewing position wherein the mirror can reflect light from a subject to a fucusing screen for focusing and framing, and then the between-the-lens shutter is again opened, creating a viewfinder system for focusing and framing.
In the above-described type of reflex camera, it is known to use an incorporated electric motor for swinging up and down the mirror and the light-blocking plate in sequence. Such a motor-driven mirror and light-blocking plate can avoid the provision of spring means for creating a driving force for a reflex mirror with its associated members which requires to be biased by a manually operable mechanism. Therefore, the provision of the electric motor makes it possible to provide a simple and compact camera mechanism.
In the case of using the motor-driven mirror and light-blocking plate, there is an interlocking mechanism between the electric motor, and the mirror and light-blocking plate for sequentially controlling the swinging up and down movement of the mirror and light-blocking plate. This interlocking mechanism considerably reduces the load exerted on the motor upon starting and distributes the load timewise, thereby leveling the load on the motor so that the motor is driven with a relatively uniform speed of rotation. As a result, the light-blocking plate can be swung up and down with considerably reduced vibration and shock.
A problem with which conventional reflex cameras with motor-driven mirror and light-blocking plate is attended is that the motor should be controlled precisely to stop according to moved positions of the mirror and the light-blocking plate. For this reason, there is provided in the camera means for detecting the position of the mirror, which comprises a photosensor adapted to detect the mirror itself. In order to detect the mirror in the viewing position and also in the photographing position, it is necessary to provide two position-sensing devices, one for each position. If the light-blocking plate must also be detected as to its position, then at least two more position-sensing devices are required. These position-sensing devices have to be precisely positioned relative to the positions in which the mirror and the light-blocking plate are to be stopped. Therefore, a camera with such position-sensing devices is difficult to assemble.
Another problem is that, although it is desirable to move the mirror and the light-blocking plate in sequence in such a way that the motor is subjected to only a small load upon starting and a distributed load timewise, the motor upon starting is apt to be subjected to sharp loads in the case of the sequential application of load to the motor for moving the mirror and the light-blocking plate. Specifically, if the light-blocking plate is moved after the completion of the movement of the mirror, the motor increases its speed of rotation at the end of the mirror movement because the load is released from the motor. Consequently, when the motor rotates with increased speed to move the light-blocking plate, there is apt to arise shock and vibrations in the motor. Furthermore, if it is attempted to move the mirror and the light-blocking plate as soon as the motor starts, their movement is actually apt to be intermittent because of the low torque of the motor at starting, resulting in undesirable vibrations of the mirror and the light-blocking plate.
These vibrations of the mirror and/or the like result in unintended movements of the camera, such as continuously making exposures. In order to avoid these vibrations of the mirror and/or the like, there is provided in the camera a mechanical braking or anti-shake device. However, such a mechanical device is not sufficiently effective in a camera with a motor-driven mirror and light-blocking plate.